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I am really strugle what neutral point represent and what it depends on..

In this link we can see how neutral point is calculated.Thay use wing and tail areas and distance between aerodynamic center (L) to determine neutral point position (D).Calculation is set like tail produce lift..(So calculation is the same like you must find fulcrum at see saw..)

What if tail produce downforce(what is often case).If this tail in my example produce downforce,neutral point will move to the left,completly different position.

Does neutral point dependt at change of aerodynamics forces at wing and tail,for example when you change tail incidence,etc?

enter image description here

If I put aircraft in vertical position to eliminate torque by gravity,connect with joint so he can rotate, now must find place at plane which will not rotate(moment is zero) when is subject to airflow.

Is this place neutral point?

enter image description here

1 Answers1

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The neutral point is the lengthwise location of the moment reference point where the pitching moment will not change with a change in angle of attack. It can be defined for a wing alone or for the whole airplane.

Of course will a change in the incidence of the tail also change the pitching moment around the neutral point. But that is besides the point: The special characteristic of the neutral point is its neutrality regarding to changes in angle of attack.

Mathematically speaking: If the slope of the pitching moment over angle of attack is zero, the reference point for this moment is the neutral point. If you pick a different point ahead of the neutral point, the slope will become negative (which characterizes a stable configuration) and vice versa. The neutral point depends on the derivative of the pitching moment over angle of attack, so the absolute value of it doesn't matter. It's the change with angle of attack that counts.

If the center of gravity coincides with the neutral point, the airplane has neutral longitudinal stability. Hence the name.

This all is valid only in the range of attached flow. If there is massive separation, all bets are off.

Peter Kämpf
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  • Neutral point must change if tail produce downforce or lift?.Calculation in my post use wing and tail planform area and distances between AC to find neutral point position.It is same calculation if two people are on see saw and you must find where is fulcrum located. –  Jan 17 '21 at 11:19
  • @EBV821, no, because it does not depend on what lift is produced at any specific angle of attack, only how it changes when the angle of attack changes. – Jan Hudec Jan 17 '21 at 11:33
  • @JanHudec So neutral point is not place where moment caused by all aerodynamics forces is zero?So if I hold plane at neutral point and negelct torque by gravity, plane will rotate about that place?(look at my "test" at bottom of post) –  Jan 17 '21 at 11:41
  • @EBV821 most planes would rotate, yes. – Jan Hudec Jan 17 '21 at 11:52
  • @PeterKampf Mathematically speaking: If the slope of the pitching moment over angle of attack is zero For what range of AoA?if AoA is 45 or 90 degrees,there will be huge change in pitching moment around neutral point..Isnt it? –  Jan 17 '21 at 11:55
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    @EBV821 for the range of AoA where lift grows linearly with AoA, i.e. approximately before onset of stall on either surface. – Jan Hudec Jan 17 '21 at 11:57
  • @JanHudec "most planes would rotate, yes" Every plane which produce tail downforce will rotate in my "test".Plane will not rotate if tail produce enough lift to match " fulcrum point".. –  Jan 17 '21 at 12:00
  • @EBV821, yes, every plane that produces downforce would. But you could construct one that is neutrally stable and that wouldn't. Nobody does that in practice because static stability is useful in most cases and distinct unstability is useful in the rest. – Jan Hudec Jan 17 '21 at 12:07
  • @EBV821 You drive me nuts with your many questions. Please read the answers more carefully. – Peter Kämpf Jan 17 '21 at 14:09
  • @PeterKämpf is it fair to describe it as kind of "vertical weathervane effect", with the a/c C of G being the "pivot axis" of a weathervane, and the NP being equivalent to the aerodynamic center of the weathervane's fins? I know it's an oversimplification and maybe it just leads to another rabbit hole, but for the narrow purpose of describing the pitch reaction to changes in AOA it might help with peoples' mental visualizations. – John K Jan 17 '21 at 17:21
  • @JohnK: Yes, the weathervane analogy is correct. Wing and tail produce lift to compensate for gravity, but that constant force will drop away when looking at the derivative over AoA, because only the changes count, not the absolute value of the forces. – Peter Kämpf Jan 17 '21 at 18:43
  • @EBV821, I think you may be confused due to not fully understanding difference between static forces and their balance, and dynamic rates of change of those forces. Imagine a ball bearing with a hole drilled through it, sliding on a wire. If the wire is shaped line a mountain, there is a single point (at the top) where it will be balanced and all forces are stable. Any disturbance will cause it to slide down the hill. Because the CHANGE in force as you move the ball slightly, is in the same direction as the motion. – Charles Bretana Jan 18 '21 at 15:10
  • If the wire is shaped like a letter "U", otoh, a slight disturbance will correct itself, as the CHANGE in the force is in the opposite direction from the resultant motion. Now add a strong magnetic field pushing (or pulling) on the ball bearing along the wire. The magnetic force would change as the ball moves closer to or further away from the magnet.. The CHANGE in the forces as the ball moves is what would determine the neutral point, NOT the actual forces. This is analogous to the CHANGE in aerodynamic forces as the AOA changes from small changes in aircraft trim or power, or whatever. – Charles Bretana Jan 18 '21 at 15:14
  • @CharlesBretana Thanks.. I find here stability lectures and explantions: https://nptel.ac.in/courses/101/104/101104062/ ........ https://www.youtube.com/watch?v=IeFGNJOR9eU&t=753s&ab_channel=BenEvans –  Jan 18 '21 at 15:55